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ALEXANDER  VOK  HTJMBOIJ>T.                173
of  real   phenomena   and   ideal views.,  reflected and
mirrored in the entirety of nature ; he brings darkly-
surmised   truths   on   the   self-discovered    basis   of
geognosy.    While we know more of the interior (the
weight, volume,  and density,)   than the  exterior  c
other planets, we had studied only the surface of the
earth, and Humboldt was the first to open the creative
laboratory of the interior of the earth to the investiga-
tions of science.    By means of natural fissures, shafts,
and mines, we know the thickness of the superficial
strata  of the earth, but the greatest perpendicular
depth which has yet been reached does not exceed
2000 feet, that is to say, only about one-ten-thousandth
part of the semi-diameter of the earth.    The masses
which the volcanos eject, and which, for the most part,
resemble  the superficial rock, must, without doubt,
come from sixty times greater depths than those which
have hitherto been explored by man.    Some sinkings-
prove that coal-beds, with their antediluvian remains^
lie (for instance, in Belgium) 5000 or 6000 feet below
the present level of the  ocean,  and that mountain
limestone has probably double that depth.    Add to
this the mountain tops as the most elevated portion
of the  surface,  and  we have a  difference of about
37,000 feet, or nearly -^^ of  the semi-diameter of
the earth.
No more than this is known of the thickness of the
earth; and the bottom of the t>cean, felt at some spots
(but frequently not fathomable with a line 25.,400
feet long) is perfectly unknown. Hence the mass of
the whole planet and its mean density, can only be
deduced from comparison of the upper attainable
portions. Nothing which lies below the above-
named depth has been examined. Nothing is known
of the depths where rocks are still fluid, of the cavities
filled with elastic vapours, of the condition of the
fluids under the pressure of confinement, of the law
of the increase of density, from the peiipherium to
the centre.
Humboldt acknowledges all this, and yet has giveit